This theory of elasticity says the extension of a spring is proportional to the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law is useful are known as linear-elastic or "Hookean" materials.

When x = 0, the spring is at the equilibrium position. This equation only works on a linear spring. A linear spring is a spring that is only being pushed or pulled in one direction, such as left or right or up or down.

Elastic potential energy is the energy saved in an object that is stretched, compressed (compression is pressing objects together), twisted or bent. For example, an arrow gets the elastic potential energy from the bow. When it leaves the bow, the potential energy turns into kinetic energy. The equation of the elastic potential energy is: